1. Field of the Invention
This invention relates to an improved process for removing heavy metal ash components from an aqueous soot suspension. More specifically, the heavy metal ash component is removed from the aqueous phase, which has been separated from the suspended soot during the standard treatment of an aqueous soot suspension, by directing the aqueous phase to a sedimentation stage for settling and subsequent removal from the aqueous phase.
2. Description of the Prior Art
Synthetic gas may be produced through the partial oxidation of oils having varying consistency and composition using oxygen or air and steam. One of the processes used over the past 25 years is the Shell heavy oil gasification process. (See Gas - und Wasserfach, 19 (1964), 512ff.)
In this noncatalytic partial combustion process the reactor temperatures range from ca. 1,200.degree. to 1,600.degree. C., and pressures preferably range from 40 to 60 bar. The recovery of the heat of the sooty synthetic gas takes place in the waste-heat boiler, to which are attached the washing plant for the removal of soot and the soot separating plant.
The amount of soot produced during gasification, ca. 1-2% of the feed oil, is precipitated with the circulating water and removed in the form of a soot slurry, e.g., in accord with DE-PS No. 1 076 092. In the preferred embodiment an organic adjuvant that is not miscible in water and that is capable of holding the soot, e.g., mineral oil, is added to the aqueous soot suspension. (See for example DE-PS No. 1 042 793.) The soot water is purified through mixing with hydrocarbons, whereby lighter hydrocarbons form a floating soot-hydrocarbon mixture, also known as soot gasoline, while the heavier hydrocarbons form soot-hydrocarbon agglomerates. The soot-oil pellets formed during mixing under stirring are removed using vibrating screens. The purified water is then returned to the circulating water system of the heavy oil gasification plant in the form of scrubber feed water. Formerly the pellets were used mainly for undergrate firing. Today the soot pellets are mixed if possible with other oil, homogenized, and added once again to the feed oil. If the soot were to consist of pure carbon, a 100% recycling of the soor would in theory be possible.
Unfortunately, however, the soot contains ash components comprising a significant amount of heavy metal compounds especially sulfides and oxides of mainly vanadium, nickel and iron which are present in the feed oil. During the recycling of the soot back to the feed oil, these ash components build up in the feed oil, the soot water, the circulating water, the waste water and the soot pellets. Higher concentrations of ash in heavy oil gasification lead to such deleterious effects as damaged reactor linings and obstructions in the waste-heat boilers. ln addition to the ash level in the circulation systems of the Shell plant, there is also an increase in the metal-carbonyl transfer to the connected gas purifiers which leads to operating disturbances and resulting in sulfide precipitation. The impact of these problems lead to more serious consequences such as plant shutdowns and expensive cleaning operations. As a result, a higher rate of soot recycling is impossible. A portion of the accumulating gasification soot, or pellets, must be removed from the plant, underfired externally, or sold. Even comparatively low recycling rates (&lt;50%) are already leading to an increase in the level of metal in all parts of the circulating system and, in turn, burdening the gasification reactors as well as the connected parts of the plant and the waste water facility.
With the transition to increasingly heavier feed oils, there has been an increase in the introduction to the ash gasification reactors constituents rich in heavy metals. Since the heavy metal compounds following the soot water separation were detected in nearly quantitative levels in the soot hydrocarbon phase, it was assumed that the ash components were incrusted in the soot. This assumption was based on the fact that it was not possible to separate the ash component in the soot using the practical methods acquired in the soot water separation, such as the use of filters, e.g., cloth filters, filter candles and coarse sand filters.
The production of pellets with reduced levels of heavy metal compounds has been accomplished by the treatment of the soot suspension with air, oxygen or mechanical action. Following the removal of the soot from the soot water, the bulk of the ash components remain behind in concentrated form in the discharge water. However, proposed methods for the separation of such ash components contain only vague data or general references to standard filtration techniques, particularly with respect to the problem of separating the most finely dispersed ash particles. The separation of the ash components from the discharge water is, nonetheless, imperative, because this water otherwise would be returned to the circulation system with ever increasing amounts of metal and in the end this would again lead to an incorporation of ash in the pellets. The separation of the ash component from the discharge water is also necessary for ecological reasons and to protect the connected parts of the plant. The use of conventional filters and centrifugal separators to achieve this end is not cost effective because of the low concentration of the ash components accompanied with the large quantity of filtrate. Coarse sand filters offer only a partial solution to the problem, since the filtered substance must be driven from the filter through back flushing, whereupon it is again present in a very finely dispersed state and in very low concentration in a large volume of water.
The above discussed problem of the buildup of heavy metal ash in the soot and water circulating systems has been generally known since the introduction of the Shell process some 25 years ago. Despite worldwide efforts no one has successfully solved the problem. High recycling rates (&gt;80%) have so far been maintained only for short periods.